refactor(tazjin/rlox): Return index after adding operations

Change-Id: I100eb9b55ace37e5c7c878d3c224b567ee8d1e36
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3738
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/rlox/src/bytecode/opcode.rs

@@ -4,6 +4,9 @@ pub struct ConstantIdx(pub usize);
 #[derive(Clone, Copy, Debug)]
 pub struct StackIdx(pub usize);
 
+#[derive(Clone, Copy, Debug)]
+pub struct CodeIdx(pub usize);
+
 #[derive(Debug)]
 pub enum OpCode {
     /// Push a constant onto the stack.